Several processes for the hydrogenation of esters of aromatic carboxylic acids (where the expression “carboxylic acids” is understood to reference both monocarboxylic acids and polycarboxylic acids) to yield the corresponding saturated homologues (i.e., where applicable, mixtures thereof of isomers produced by hydrogenation) have been known now for almost 100 years in the background art. Many of such processes are based on heterogeneous catalysis with active metals and are performed in the liquid or gaseous phase with or without the use of reaction solvents/diluents, exposing the initial aromatic ester and a hydrogen source (almost always a hydrogenating gas, which often but not necessarily is constituted by pure hydrogen) to contact with the active metal catalyst, conducting the consequent exothermic hydrogenation reaction in semi-continuous or continuous conditions.
A common factor of such known processes for hydrogenation of aromatic acid esters is the use of rather high hydrogenating gas pressures during the reaction, especially if the reaction is performed in the liquid phase, and this occurs in most processes used on an industrial scale, in particular if related to scarcely volatile aromatic carboxylic acid esters.
For example, GB 286,201 discloses a reaction of aromatic carboxylic acid esters conducted at temperatures comprised between 120° and 150° C. and at pressures between 25 and 40 bars in the presence of nickel catalysts.
U.S. Pat. No. 2,070,770 notes that the reactions according to GB 286,201 are considered slow and not quantitative, in addition to being limited to the corresponding ethyl esters, and therefore teaches a process for the hydrogenation of dialkyl esters of phthalic acid with alcohols comprising at least 8 carbon atoms, performed at 160-260° C. and at 750 psi (51.7 bar)-5000 psi (344.7 bar).
U.S. Pat. No. 3,027,398 relates instead to the catalytic hydrogenation of dimethyl terephthalate performed at 110-140° C. at 500 psi (34.5 bar)-1500 psi (103.4 bar), in which the initial dimethyl terephthalate is dissolved in dimethyl 1,4 cyclohexane dicarboxylate (which is the saturated product) as reaction solvent.
DE 1 154 096 discloses the hydrogenation of C1-C5 alkyl terephthalate at temperatures comprised between 150° C. and 250° C. and at hydrogen pressures comprised between 20 and 300 atmospheres, with the addition of 10-25% by weight (relative to the terephthalate) of alkyl ester of p-toluic acid and/or ester of 4-methyl cyclohexanecarboxylic acid in the presence of nickel.
U.S. Pat. No. 3,334,149 teaches the hydrogenation of molten alkyl terephthalate at pressures comprised between 50 and 500 atmospheres and temperatures comprised between 100° C. and 400° C., preferably 150-275° C., on a fixed bed palladium catalyst.
DE 2 132 547 relates instead to the hydrogenation of aromatic compounds (including benzoic acid esters or phthalic acid esters and isomers) at temperatures comprised between 30° and 250° C. and at pressures in excess of 50 bars with ruthenium catalysts.
DE 28 23 165 teaches the catalytic hydrogenation of esters of aromatic carboxylic acids on a fixed bed of catalyst of nickel, ruthenium, rhodium or palladium, at temperatures between 70° C. and 250° C. and at pressures between 30 and 200 bars in the liquid phase or at the same temperature and at pressures between 1 and 10 bar in the gaseous phase.
WO 99/32427 discloses the hydrogenation of esters and anhydrides of benzene polycarboxylic acids at temperatures comprised between 50° C. and 250° C. at pressures in excess of 10 bar, comprised preferably between 20 and 300 bar, using three different types of catalyst. The reactions exemplified in WO 99/32427 are performed in the liquid phase and at pressures of 100 or 200 bar.
US52869898 concerns a process for the preparation of dimethyl cyclohexane carboxylate by hydrogenation of dimethyl benzenedicarboxylate. The preferred temperature is in the range of 140-220° C. and the preferred pressure range is 50-170 bars absolute. The catalyst employed comprises palladium and a second Group VIII metal. According to the optimum conditions detailed, the reactions are carried out in continuous modus at a pressure of 125.1 bars.
US2005/101800 concerns the preparation of cycloaliphatic polycarboxylic esters by hydrogenating a partial ester of the corresponding aromatic carboxylic acid or of the corresponding aromatic polycarboxylic anhydride, and, thereafter, reacting the resultant cycloaliphatic partial ester with an alcohol to give the sought full ester.
WO 94/29261 reports on the low yields obtained applying the processes described in older patent literature when the hydrogen pressure employed for the hydrogenation of dimethyl terephtalate is lower than about 135 bars absolute. Before this background, WO 94/29261 proposes a continuous process for the manufacture of a cyclohexanedicarboxylate.
WO 2004/046078 concerns hydrogenation of benzenepolycarboxylic acid derivatives to give corresponding cyclohexylderivatives. The most preferred pressures applied exceed 100 or 130 bar, with a maximum of about 300 bar. Preferably, the process is carried out continuously and in the presence of a solvent.
US 57208545 concerns the hydrogenation of novel substrates substrates which are different from diesters of phthalic acid and from triesters of 1,2,4-benzene tricarboxylic acid
It is thus evident that the pressures of hydrogenating gas commonly used in the background art to saturate polycarboxylic acid esters in the liquid phase exceed on average 20-30 bar, often exceed even at least 50 bar, and therefore require an adequate mechanical sizing of the equipment used, which must withstand such pressures, and must be protected adequately against the risk of hydrogen embrittlement.
On this background, in the background art there is the need to provide new processes for the liquid phase hydrogenation of aromatic carboxylic acid esters that do not entail these disadvantages and therefore provide improved or alternative access to the respective hydrogenated products.
This occurs because in recent times cyclohexanecarboxylic esters, particularly esters of hexahydrophthalic acid with C1-C16 alkanols or mixtures thereof, have generated considerable commercial interest as products that can be used potentially to replace at least partially certain phthalic plasticizers that are traditionally used widely in plastic materials but were subjected to restrictions by the European Union in 2007, for example as regards use in toys, in articles for babies and in articles intended for contact with food. Potential risks arising from phthalic plasticizers that are widely used have also been hypothesized for medical articles and devices (blood bags, enteral nutrition kits, etc.). Several phthalic plasticizers, such as for example di-2-ethylhexyl phthalate (identified below as “DEHP” and also sometimes termed less specifically in the literature as DOP—di-ocytl phthalate), di-isobutyl phthalate (DIBP), and n-butyl phthalate (DBP), are therefore currently included in the so-called “SVHC List” (Substances of Very High Concern) prepared by the European Chemicals Agency (ECHA). For this reason, the assessment of replacement of these substances with other alternative ones that do not have toxicological problems is already in progress. Consequently, the identification of molecules that are alternative to those included in the SVHC list is of considerable interest for modern chemical industry.
Necessarily, it is essential that the identified molecules have technical characteristics that are comparable to the extremely advantageous and particular ones of traditional phthalic plasticizers. As regards for example the processing behavior characteristics (or “workability”) of esters of cyclohexanedicarboxylic acids, the “PVC Handbook” by Charles E. Wilkes et al. (ISBN 3-446-22714-8), in its 2005 edition, predicted on page 185 that for example di-isononyl cyclohexane 1,2 dicarboxylate (HDINP), a hydrogenated product that is “homologous” to traditional di-isononyl phthalate (DINP), recently introduced on the market for some of the most sensitive plastic material applications in the pediatric and medical field, will have a performance that is substantially similar to the respective phthalate, except for the expectation of a reduced dissolving power of the polymer where it comprises PVC, which can be deduced from the loss of the aromaticity of the ring in di-isononyl cyclohexane 1,2 dicarboxylate.
However, it was found subsequently that the differences between phthalate and direct hexahydrophthalic homologue can be even considerable in practice, especially as regards the processing behavior of the mixtures between plasticizer and polymer. The former is of interest since the processing behavior, especially the incorporation time of the plasticizers into the most employed types of polymer preparations (such as e.g. dry-blend or plastisol) is a critical parameter for the efficiency (either in terms of energy consumption or output) of the overall polymer working process aiming at the provision of objects made from plasticized polymer.
Thus, for example, a study entitled “Plastisol ReFlex™ 100 Evaluation” published in May 2011 by the Pasadena Plasticizer Application Lab (PPL) on the Internet for PolyOne at the address http://www.polyone.com/en-us/docs/Documents/Plastisol_reFlex %28TM %29_100_Evaluation.pdf teaches that in order to obtain processing behavior characteristics that can be compared to the use of pure DINP as plasticizer (in particular in order to obtain the same processing temperature of the respective PVC plastisols) it is necessary to use mixtures between 65% by weight of di-isononyl cyclohexane 1,2 dicarboxylate (HDINP) and 35% by weight of epoxidized fatty acid monoester, since the melting point of the plastisols prepared only with di-isononyl cyclohexane 1,2 dicarboxylate (HDINP) exceeds by as much as 15° the fusion temperature of conventional plastisols, i.e., those that contain the same quantity of pure DINP.
Therefore, the recent study by PPAL confirms that the processing behavior of di-isononyl cyclohexane 1,2 dicarboxylate (HDINP) in PVC is actually reduced with respect to DINP, a fact which leads subsequently, for example, to a reduced production rate and/or to a higher energy expenditure in plastisol processing applications.
Besides the comparison with its direct “homologue” DINP, the industrial applicability of di-isononyl cyclohexane 1,2 dicarboxylate (HDINP) on a more general scale, potentially almost a universal one, is to be evaluated also—in view of its compatibility with an already broad and rapidly growing range of toxicologically safer products—with respect to DEHP, which is instead the current plasticizer of reference in very wide generic use. According to the above cited PVC Handbook (see in particular Chapter 5.4, “Types of plasticizers”, specifically the discussion of phthalates on page 177 and table 5.2 on pages 179-180), di-2-ethylhexyl phthalate (DEHP) is used historically as a common reference standard to evaluate the performance of all other plasticizers, both phthalic and otherwise, and is used further as a standard to conceive theoretically mixtures of plasticizers for specific applications.
As explained below in the present application, the inventors of the present invention have been able to confirm, as part of their studies, that di-isononyl cyclohexane 1,2 dicarboxylate (HDINP) is inferior, as regards some particular characteristics related to processing, specifically to its incorporation time in the polymer or in the polymeric mixture, such as the dry blend time or the gel time or fusion time, both to DOP (or DEHP), i.e., the current plasticizer of reference, and to DINP (its direct aromatic homologue), especially as regards applications for PVC.
It is therefore evident that the hydrogenation of the aromatic system in certain aromatic plasticizers, more concretely the transition from DINP to HDINP, can compromise the processing behavior of the resulting polymer/plasticizer mixture; in particular, hydrogenation can compromise the processing behavior in dry blends and in plastisols of PVC, and consequently can lower the production rate in PVC applications. This loss of performance in PVC can constitute, at least for less sensitive generic applications (i.e., of the general-purpose type, as classified by the PVC Handbook), especially if subject to large-scale production, an obstacle to a wider establishment of di-isononyl cyclohexane 1,2 dicarboxylate (HDINP) or similar products on the market.
As regards instead special or very special applications (so-called “specialty plasticizers” (SP), according to the PVC Handbook), it should be noted that in some fields of technology, for example where plasticizers with extremely low diffusivity are required (see SP-LD plasticizers, again as in the PVC Handbook), plasticizers are already in use which are free or substantially free from phthalates and therefore are already compatible with 2007 UE standards et seq.
This is the case, for example, of trimellitates, which are triesters of 1,2,4-benzene tricarboxylic acid (comprising, for example tri-2-ethylhexyl trimellitate or TOTM, tri-isooctyl trimellitate or TIOTM, tri-n-octyl trimellitate or TM8, tri-isononyl trimellitate or TINTM, tributyl trimellitate or TM4, trimellitate of C7-C9 linear alkanols or TM7-9, trimellitate of C8-C10 linear alkanols or TM8-10 and others) and are distinguished—with respect to the reference DEHP—by extremely reduced volatility and higher resistance to extraction, exhibiting at the same time excellent electrical properties.
These characteristics qualify trimellitates traditionally for use in insulators, for example for electrical cables, especially if rated for high temperatures, for use in leathers and in synthetic coverings for car interiors—and more generally for all applications in which one wishes to minimize the release of plasticizer from the mixture with the polymer, especially if exposed to heat sources (see PVC Handbook, page 330).
However, the processing behavior of plasticizers of the trimellitate class is reduced with respect to DEHP of common reference, especially if the trimellitates are used in a mixture with PVC.
Therefore, it would be desirable to identify, preferably in the field of special applications, particularly in the field of SP-LD plasticizers to which trimellitates belong, alternative plasticizers that are still free from phthalates but improve the processing behavior of the polymer/plasticizer mixtures that contain them, in particular of PVC-based plastic mixtures. This would make available new mixtures between SP-LD plasticizer and polymer, in particular new mixtures comprising plasticizer and PVC, free from phthalates but at the same time characterized by better processing behavior.
Moreover, as seen above, although the study of hydrogenation of aromatic esters is in progress since the 1920s and despite the many possibilities developed for plasticizing synthetic polymers that appeared after the Second World War, in the background art there is still considerable interest
in identifying new advantageous technical applications of plasticizers (or mixtures thereof) with reduced toxicity, preferably free from phthalates, applications that entail performances comparable or improved with respect to traditional plasticizers,
in providing new processes for the hydrogenation of esters of aromatic carboxylic acids, as an alternative or as an improvement to known hydrogenation processes, in order to facilitate the provision of plasticizers with reduced toxicity and thus have available additional sources of said plasticizers that can be used for the above-cited new applications, and
in providing new plasticized polymeric mixtures comprising polymers and plasticizers with reduced toxicity, particularly free from phthalates, that have improved applicability.
The aim of the present invention is therefore to solve the problems observed in the background art.